Field of the Invention
The present invention relates to an electron beam sterilization unit for processing food packaging material.
Description of the Related Art
In the food industry, and particularly in the packaging of numerous pourable food products, such as fruit juice, milk, wine, etc., automatic packaging machines are used to produce aseptic packages from sheet packaging material, which is normally in the form of pre-cut blanks or a continuous strip and is subjected to a series of longitudinal folding and sealing operations to form a continuous tube of packaging material which, once filled, is sealed and cut transversely into individual packages.
Before it reaches the packaging machine, the sheet material is fed through a sterilization unit, on which it is processed to destroy microorganisms, to ensure the packaged products are microbiologically safe, and to effectively preserve their organoleptic and nutritional properties.
For this purpose, various types of sterilization units have been devised, the most advantageous of which are those employing electron beams to irradiate the two opposite faces of a moving web of packaging material.
Known electron beam sterilization units normally comprise a frame; a channel formed through the frame to guide the web of packaging material in a given direction; and two electron beam emitters fitted to the frame, on opposite sides of the web guide channel. Each emitter comprises, in known manner, a vacuum tube in which the electrons, emitted by a cathode, are accelerated by an electrostatic field and are shot out of the tube, in the form of electron beams, through an irradiation window, closed by a thin plate normally made of a foil of titanium, aluminium, silicon, etc., to strike a respective face of the packaging material as this moves along the guide channel.
An important and highly delicate aspect of known sterilization units of the type described above is the way in which the emitters are locked to the frame. In fact, it is essential that the system for assembling and locking the emitters to the frame should allow for installing and removing them easily and safely whenever they need changing or servicing, and for also locking the emitters in the work position quickly and easily, but at the same time in controlled manner, to avoid endangering the integrity of the emitter and in particular the delicate foil closing the electron beam irradiation window.
A sterilization unit of the type described above is known, for example, from the Applicant's International Patent Application No. WO2004/111469, in which the locking system comprises, for each emitter, a cam device, which, by moving an operating member, moves a pusher crosswise to the travelling plane of the material. The pusher in turn acts on a flange on the emitter, facing the channel, to push and seal the flange onto a portion of the frame and so lock the emitter in the work position.
Though effective, the locking system described above has several drawbacks, due to the cam device failing to ensure safe, irreversible grip in any stress condition, and due to the design of the locking system whereby, once the flange is fastened onto the frame, the rigid connection formed between the flange and the frame allows no compensation of any ‘static’ deformation of the flange caused, for example, by unevenness of the gripping surfaces, or of any ‘dynamic’ deformation of the flange caused by in-service thermal expansion. This may result in stress within the body of the emitter, which in turn may result in bending of the foil closing the irradiation window, and therefore in malfunctioning of or damage to the emitter.